Some liquids such as draught beverages require certain levels of gases, particularly carbon dioxide, alone or together with other gases, to be dissolved in at least one of the constituent liquids in order to achieve a desired property such as the desired taste and presentation effects in the dispensed drink. Other liquids such as certain dairy products similarly require levels of nitrous oxide, alone or together with other gases to be dissolved in at least one of the constituent liquids in order to achieve a desired foamed constituency upon dispense
The use of gas/liquid contactor modules containing gas-permeable hollow fibres for controlling dissolved gases in liquids is well known. Examples of such contactors and associated schemes for control of their operation have been described in U.S. Pat. No. 5,565,149 and U.S. Pat. No. 7,104,531, the disclosures of both of which are incorporated herein by reference. The advantage of such contactors is their capability of achieving bubble-less and efficient transfer of gases into solution in liquids without causing turbulence or mechanical agitation of the liquid.
These contactor modules are typically constructed with a gas port which is connected to a pressurised gas source and two ports connected respectively to a liquid source and to a dispense tap.
For contactor modules utilising the types of fibres described in U.S. Pat. No. 5,565,149 and U.S. Pat. No. 7,104,531, the gas port communicates with the cores or bore side of the hollow fibres, and the liquid ports communicate with the outer surfaces or shell side of fibres. This format provides a large surface area for contact between gas and liquid to give efficient gas transfer into the liquid together with low frictional loss when liquid flows through the contactor. The gas transfer efficiency, defined as the ratio of gas actually dissolved in the output liquid to the saturation level of gas for the applied gas pressure and the process temperature, depends on the detailed design of the contactor. This efficiency generally increases with increasing residence time of the liquid within the contactor.
The preferred type of fibre used in such contactors may be classified as permeable, asymmetric skinned, and hydrophobic. Such fibres are preferred for the addition of gases to beverages because they have a relatively high resistance to flooding and their surfaces which are in contact with the liquids are smooth and so contain very few sites which could encourage biological growths to form. However, in practice, there may be a small number of physical defects in some fibre walls which defects may allow passage of liquid from the shell side into the bore side when hydraulic pressure exceeds gas pressure. The rate of liquid penetration through such defects increases in proportion to the pressure differential between liquid and gas.
Beverage dispense applications involve long periods when liquid is static within the shell side of the contactor before being caused to flow out to the dispense tap. Practical dispense systems using hollow fibre contactors therefore include pressure control devices in the feed gas and liquid streams to avoid flooding of the fibre bores and also to ensure that the liquid retains the gases in solution within both the contactor and the tube leading from its outlet port to the dispense tap. U.S. Pat. No. 5,565,149 and U.S. Pat. No. 7,104,531 disclose examples of such controls.
In practical tests with these contactors, liquid can be detected in the bore side of modules after maintaining an excess liquid pressure of about 0.1 MPa (1 bar) to the shell side for longer than 1 hour. Ultimately, exposure to such condition will cause the bore side volume of some fibres to flood and lead to reduced efficiency of gas transfer.
Standard pressure control devices can be used to achieve an approximate balance between liquid and gas pressures for contactors when the liquid is supplied from a gas-driven pump. It is also possible to achieve approximate pressure balance when electrically-driven pumps are used.
It is a natural characteristic of electrically-driven and gas-driven beverage pumps that their liquid delivery pressures increase when output liquid flow rates are reduced, and are at a maximum when the outlet flow is stopped.
In beverage dispense applications this characteristic is exploited to cause such pumps to stop and start automatically in response to their downstream liquid pressures. Most electrically-driven pumps for use with beverages incorporate a pressure switch communicating with their outlet for liquid delivery, while gas-driven pumps rely on flexible diaphragms and non-return valves. Working differentials between the starting pressure and the stopping pressure of these pumps are due to the mechanical hysteresis in their corresponding components, so that, when the dispense tap is open, the liquid's pressure at the outlet of the pump is lower than when the dispense tap is closed.
With conventional control schemes in beverage dispense systems using membrane contactors, the pressures of gas and liquid within the contactor can therefore be balanced with reasonable accuracy either for the condition when liquid is flowing or for the condition when it is not flowing.
In draught dispense practice, since liquid is only caused to flow intermittently through the contactor, controls will conventionally be chosen to protect the contactor by arranging for the pressures of gas and liquid to be balanced during the much longer periods when there is no requirement for liquid to flow. Consequently, the applied gas pressure will normally be greater than the applied liquid pressure during dispense flow.
For a well-designed membrane contactor, this method of control exposes the carbonated liquid to super-saturated conditions during dispense, risking the formation of gas bubbles in the contactor and in the tubing between its outlet and the dispense tap. Super-saturation increases the difficulties of dispensing highly-carbonated beverages, especially those which have a tendency to form foam on dispense. Examples of such drinks include beers, lagers, wines and some brands of whisky-water mixes.
U.S. Pat. No. 5,565,149, in the FIGS. 10 and 11 and in the description in that document, disclosed for the first time the observation of surprisingly high carbonation levels when carbonating beverages in certain types of dispense systems utilising membrane contactors. In U.S. Pat. No. 5,565,149 it was postulated that intermittent operation of the dispense tap caused transients in the pressure and flow in the liquid side of the contactor which resulted in significant changes in liquid boundary layers surrounding each fibre, and hence allowed an increased carbonation compared to that found under operation at continuous liquid flow.
In parallel with that surprisingly increased carbonation, the pressure of liquid within the contactor increased after the dispense tap closed, and in U.S. Pat. No. 5,565,149 it was assumed that this pressure increase was the result of the increased carbonation.
In our tests of beverage dispense systems using membrane contactors to carbonate liquids we have now discovered that the explanation for that surprising observation given in U.S. Pat. No. 5,565,149 was incomplete.
We have found that this increase in liquid pressure will occur in all beverage dispense systems using membrane contactors to dissolve gases in liquids where, at the end of each dispense event, the liquid-containing part of the contactor communicates with a closed liquid volume.
This effect has an important and additional significance in the control of such systems utilising such contactors.
It is our present belief that, heretofore, no control systems have been commercially available that are capable both of protecting the membranes of dispense systems utilising membrane contactors of the general type generally described in U.S. Pat. No. 5,565,149 from flooding during standby periods and of avoiding super-saturation during dispense events. Any such control systems would need to have parts that contact the liquid being dispensed that can be sanitised in situ using normal cleaning procedures.
Using a conventional control scheme, at the instant when the dispense tap is closed the pressure of liquid within the contactor increases as expected to the normal stalled pressure which is characteristic of the particular beverage pump being used. However, we have found that this liquid pressure does not then remain constant, but starts to increase further over a short period of time. The final pressure achieved is significantly greater than the pump's stalled pressure, and it then remains constant until the next dispense event.
Our measurements show that the magnitude of this effect is very similar for carbonation of beer, wine or de-aerated water at a temperature of 3 degrees Celsius, using membrane contactors with liquid capacity 200 ml.
We have carried out detailed measurements using deaerated water as the liquid being carbonated, the results being set out below.
Liquid volumes each of 250 ml were dispensed using a flow rate of 11 ml per second at equal intervals of 2.5 minutes. From previous measurements it was determined that the efficiency of the contactor being employed was approximately 93% for a continuous flow at 11 milliliters per second. The residence time of 2.5 minutes between dispense events is known to be sufficiently long for the contactor's liquid contents to reach full saturation.
Liquid was supplied to the contactor by a gas-driven pump connected to a gas pressure of 0.25 MPa, which resulted in a flow pressure of 0.22 MPa and a stalled pressure of 0.25 MPa. In these measurements the pressure of carbon dioxide applied to the contactor was maintained constant at 0.22 MPa.
Each time that dispense flow stopped, the observed liquid pressure in the contactor immediately increased from 0.22 MPa to the stall pressure of 0.25 MPa, and then it began to increase further over a period of 25 seconds and reached a final value between 0.33 and 0.35 MPa. This pressure then remained constant until the next dispense event.
This effect, causing a significant increase in liquid pressure above the beverage pump's stalled pressure, means that previously proposed draught carbonating dispense systems utilising membrane contactors have been unable to achieve the necessary controlled balance between liquid pressure and gas pressure, with the consequence that the efficiency of the membrane contactor decreases over time.
In further tests carried out on the system using the same process conditions, after each closure of the dispense tap, additional amounts of liquid were carefully withdrawn downstream of the contactor. These amounts were small enough to prevent the gas-driven liquid pump from re-starting. The liquid pressure in the contactor was initially reduced by this action, and then increased at the same rate and over the same period as previously observed, but to final values which were lower than observed in the earlier tests.
We have found that the final liquid pressure was determined by the volume of extra liquid drawn off, provided that such volume was less than 0.9 ml. When the additional volume of 0.9 ml or greater was withdrawn the final liquid pressure achieved was equal to the applied gas pressure.
In yet further tests, the single contactor was replaced by two contactors of the same type which were connected in series. Process temperature and pressure conditions were unaltered, but the dispense volumes and flow rate were increased to 500 ml and 22 ml per second respectively. We found that a final pressure balance could be achieved if the amount of liquid withdrawn after dispense was increased to 1.8 ml.
Carbonation of water is known to be an exothermic process, and at this level it causes approximately 2 degrees Celsius increase in liquid temperature. However, this would result in a thermal expansion of only 0.08 ml of liquid within the single contactor used in our tests. We therefore conclude that the observed effect of pressure increase was not caused by thermal expansion of the liquid.
At the instant when the dispense tap closes and liquid flow stops, a gradient in the local carbonation naturally exists within the contactor, with virtually no carbonation present at the inlet end and a high carbonation, here at 93% of saturation, at the outlet end.
The ultimate carbonation level of the liquid within the contactor, attained a short period following closure of the dispense valve, is determined by both the applied pressure of gas and the temperature of the liquid.
We concluded that net expansion of the liquid was caused by the process of additional carbonation taking place within the contactor, commencing at the instant when dispense flow stops and continuing until all of its contained liquid reaches saturation carbonation. The hydraulic pressure of the trapped liquid increases as it expands against containment by membrane fibres and flexible tubing in the circuit.
We are supported in these views by experimental work published in a quite different field bearing no relation to carbonating beverages during dispense utilising membrane contactors, namely ocean research. Yongchen Song et al have shown that the ratio of density of carbonated water to that of plain water, and the difference between those densities, increase linearly with the level of carbonation, and that these effects are independent of pressure and temperature. (Measurement of the density of CO2 solution by Mach-Zehnder Interferometry; Yongchen Song et. al.; Annals of the New York Academy of Sciences 972 (2002); 206-212).
The magnitudes of the volume increases which we found in our own tests described above are in full agreement with calculations from the published data of Yongchen Song et al.
We have found that, in carbonation dispense using membrane contactors, the amount of this liquid expansion is proportional to the liquid-containing volume of the contactor and also to the ultimate saturation level of carbonation.
For carbonation, the magnitude of the expansion is simply expressed to sufficient accuracy by formula (1) below:Δv=K·Vc·C·(1−0.5η)  (1)where
K=a constant, approximately 7.2×10−4 
Δv=characteristic liquid expansion amount for the contactor, in milliliters
Vc=liquid volume of contactor, in milliliters
C=saturation level of carbonation, in grams per liter
η=efficiency of contactor at continuous flow condition
Many carbonation dispense applications require relatively high flow rates, say 0.045 liters per second or more, and relatively high carbonation levels, say 10 grams of dissolved carbon dioxide per liter or higher. In order to achieve such carbonating performance the contactors will have liquid volumes of the order of 0.5 liters. The amount of liquid expansion following closure of the dispense tap will therefore be greater than 2 ml. This expansion will cause a very significant increase of liquid pressure, especially for compact carbonating systems of the type that would be employed in dispensing beverages from a bag-in-box container such as a polypin container employed for beer, with consequent damage to the membrane contactor.
Similar relationships will apply for other gases than carbon dioxide, but with different specific values for the constant K.
A significant expansion effect will result when using other gases, such as nitrous oxide, which, like carbon dioxide, have high solubilities in the liquids which form constituents of beverages.
Thus, problems similar to those discussed above will arise in dispense systems for other liquids or semi-liquids that add a highly soluble gas to the liquid at the point of dispense, where a membrane contactor is employed, as for example in the dispensing of foamed milk or cream, where the gas added at dispense is nitrous oxide. Where the added gas is nitrogen, oxygen or mixtures thereof such as compressed air, the problem is not significant, since the solubility of these gases in an aqueous liquid is very much less than the solubility of carbon dioxide or of nitrous oxide.
The present disclosure seeks to overcome the problems inherent in previous systems involving addition of carbon dioxide or nitrous oxide to liquids during dispense utilising a membrane contactor.